Phosphoric acid has three dissociation constants.
Thus, you could make a buffer of pH 2 by preparing a solution of phosphoric acid and sodium dihydrogen phosphate (Equation 1).
You could make a buffer of pH 7 by preparing a solution of sodium dihydrogen phosphate and sodium hydrogen phosphate (Equation 2).
You could make a buffer of pH 12 by preparing a solution of sodium hydrogen phosphate and sodium phosphate (Equation 3).
The ranges for each buffer would be